Although the following description relates almost exclusively to plaques ideally adapted for use in fully pre-mix burner applications, it is to be mentioned that the configuration of the burner plaque of the present invention renders said plaque utile in different applications, for example non-pre-mix and normally aspirated applications, and the invention should not be considered as limited by the following description.
Fully pre-mixed burners are so-called because the fuel, usually gas (under denominations for reference gases and test gases identified in European Standard EN 437), and a fan supplied quantity of air exceeding the stoichiometrically correct amount of air for the specific gas type (superstoichiometric) are mixed to produce a combustible mixture which subsequently is ignited to produce a burner flame that, in the case of heating the water in a boiler, is applied to a heat exchanger of the boiler. The term pre-mixed arises therefore because of the mixing of the fuel and air before the flame strip.
There are other types of burner which operate in a mode in which a sub-stoichiometric amount of primary combustion air is mixed with the fuel before the flame strip, secondary air required for completing the combustion process being induced into the flame after ignition of the gas/primary air mixture. These other burners are known as partially pre-mixed burners. The present invention may be applicable to fan-assisted models of such burners, but its best application is to the fully pre-mixed type, as partially pre-mixed burners are limited by the relatively high levels of nitrogen oxides (NOx) they generate during the combustion process and as such, these burners are diminishing in popularity.
Fully premixed burners tend to be high intensity burners in which high volumes of gas/air mixture are forced through a relatively small plan area burner, and specifically through the ports in a burner plaque to give a compact, high intensity flamestrip which sits on or near the face of said plaque. They can be fired in any orientation and are used in most condensing boilers where the burner fires downwards into the heat exchanger.
The high volume of the gas/air flow being fan driven through the small area burner means there is a high “port-loading” on each individual burner port provided in the plaque. The fact that a compressible medium flows through the burner system at a certain velocity means that any instabilities created on ignition of said medium are amplified and can ultimately develop a common frequency which constructively harmonises with the natural frequency of the boiler system to generate a phenomenon called combustion resonance. Resonance of any audible volume or frequency is unacceptable for pre-mixed burner applications.
The boiler system comprises the combustion chamber, the heat exchanger which will occupy a predetermined position within said combustion chamber, and a flue attached to said chamber to vent the exhaust gases of combustion. Any variance of these parameters will influence the harmonics of the system e.g. varying the flue length will change the back pressure on the combustion chamber.
The combustion resonance is manifested as three distinct types of resonance:
1. A low frequency (125 to 200 Hz) rumble on ignition; This is believed to be due to flame instabilities caused by poor gas/air mixing, bad gas/air mixture distribution and poorly timed ignition, such being associated with the burner appliance design factors of upstream mixing of gas and air, position of ignitor etc.
2. A higher frequency (250 to 315 Hz) resonance on ignition at volumes up to 95 dB; Under standard repeat ignition conditions the flame ignites and thermally fluctuates initially as it stabilises near the port. The differential pressures and temperatures created initially in the system exacerbate this instability creating a range of oscillating and fluctuating frequencies of flame vibration, some of which may harmonise and thus be amplified at one or more of the natural resonance frequencies bands of the system. However, once the system has been operational for approximately a minute, these instabilities dissipate and the resonance fades out.
The propensity of boiler systems to develop resonance increases when the system is cold prior to ignition. The energy of the ignition combustion wave within the system is dissipated as it spreads away from the ignition source and comes into contact with the cold surfaces of the system, which further reduce the velocity of the flame front. This results in a temperature differential between the area close to the ignitor and at the far end of the burner giving rise to a horizontal pressure wave which interacts with the oscillating thermal fluctuations described above to destabilise the flame and also to increase the propensity for fluctuations over a broad range of frequencies. This broad fluctuation range increases the likelihood of harmonisation and amplification at one of the natural vibration frequencies of the system. This resonance is not fully temperature dependent and is directly influenced by the burner plaque and distribution plate designs.
3. A continuous high frequency resonance can develop once the flames have stabilised. This can arise from instabilities caused by ignition resonance and which are continuously excited by virtue of the gas/air flow movements within the system during operation, or by the inherent excitations developed by virtue of the burner design.